Solar desalination unit

For clear drinking water

Figure 1 Number of habitations in India with
salinity more than 1500 ìg/litre

Water, everywhere. Nor any drop to drink. This line truly reflects the water crisis that the world is facing, especially those who only have access to saline water. The fact is that almost two-thirds of the earth's surface are covered with water. Of this, 98% is saline and, therefore, not potable. Hopefully, with desalination technology picking up steam, both the statement and the reality it conveys – water scarcity – might soon be outdated.

Harnessing natural resources

Desalination processes have been used for many a decade but its high energy requirement and, therefore, their prohibitive costs have prevented their widespread adoption across countries. Researchers at TERI have found an innovative way to bypass the limitations of conventional desalination technology and solve the acute water scarcity problem of both urban and rural areas with a solar desalination unit. Designed in collaboration with the SEC (Solar Energy Centre) of the Ministry of Non-conventional Energy Sources, this desalination unit is the first of its kind in the country and the current prototype has been specifically designed to cater to the needs of remote, rural households. The prototype has already been tried and tested at the Solar Energy Centre and the results have been more than promising.

In dire straits

The severity of fresh water shortage can easily be gauged by the fights that have erupted over water in the recent past in India and outside. Statistics emphasize the same–at least 40% of the world's population lives without drinking water and roughly 80 000 habitations across the planet have no source of safe water. Of the 575 000 Indian villages, about 162 000 face problems of brackish or contaminated water, and scarcity of fresh water.

One of the major reasons for the paucity of fresh water is over–exploitation of groundwater. Levels have dropped by 5–10 metres in the arid areas of India. Overpumping, especially in the coastal regions, has led to severe salinity ingress, making water unfit for human consumption. Besides, fresh and saline aquifers are often in close proximity, and pumping out fresh water without corresponding recharging causes inflow of saline water into fresh water systems.

The other aspect of the issue is the high salinity level in fresh water sources. In urban and rural areas of India that have access to water, a sizable population has, for generations, been drinking water with TDS (total dissolved solids) levels of about 1500 to 2000 ìg/litre (microgram per litre) though the BIS (Bureau of Indian Standards) specifications for drinking water limits the amount of TDS at not more than 500 ìg/litre. India has about 53 000 habitations with salinity greater than 1500 ìg/litre, most being remote and arid areas with saline water (Figure 1).

Transforming lives

With limited stock of fresh water and an ever-increasing demand for it, the urgent need for desalinating water in India cannot be stressed more. The renewable-energyoperated desalination systems offer a feasible solution to this chronic problem. Normally water scarcity increases during summers, and at that time desalination systems also become more effective since solar radiation is higher than average. As is clear from Figure 1, most places suffering from salinity lie in high radiation zones–5.4 to 6.4 kWh/m2 (annual average). This makes the use of solar desalination systems in these areas even more practical and sustainable.

Factors like absence of electricity, heavy investment, and the polluting nature of conventional fuels rule out the possibility of working with regular desalination technologies in rural India. The current basin-type solar still is the only commercially available solar device to desalinate water. This barely delivers 2.0–2.5 litres of water at a time and costs 5000 rupees.

To get more water out of the system, more solar stills need to be coupled to each other, making the whole exercise pretty cumbersome. Also, certain inherent flaws in these devices – leakage of water vapour through joints and glass sealing of solar stills, and the delivery of a small quantity of desalinated water – pushed them into disuse. Conventional multistage desalination plants too pose problems related to their need for high temperature and vacuum.

TERI's current solar desalination unit consists of 10 flat-plate solar collectors, 4 trays that hold brackish water, and an efficient and compact heat exchanger in the bottom tray (Figure 2). The unit is designed to deliver 100 litres of water, translating to 42% more output vis-à-vis the commercially available single-basin solar still. Though the process of desalinating saline water – evaporating water, condensing and collecting pure water vapours – remains the same, what makes all the difference is its no-fuss, closed-loop design and the total absence of conventional fuels, thus making it totally sustainable, affordable, and eco-friendly (Figure 3).

Beneficiaries

The solar desalination unit holds the potential to transform the lives of thousands of people in arid villages and small coastal towns spread across the country. Each unit is capable of delivering about 100 litres of water every day. Since the system does not depend on pumps or any other active component that requires daily maintenance, the unit is easy to maintain. Though the current prototype is custom-made for rural households, it can easily be modified for industrial application.

Applications/benefits

Economic analysis establishes that the annual cost of desalinated water from the TERI-SEC prototype desalination system is less than that from a solar still. This cost-effective medium is a positive step towards providing fresh water to those condemned to drink saline water.